This invention relates to a collimating lens with an external aperture stop and, more particularly, to a collimating lens having a doublet element of both crown and flint glasses disposed between front and back positive-power lens elements for increased field of view and reduced chromatic aberrations.
Collimating lenses are used in numerous situations, one situation of interest being in an optical apparatus for scanning a display, such as a display formed of light-emitting diodes. An image of a subject on the display is transmitted via a scanning mirror, or drum, to an objective lens for a viewing of the subject by a person. Existing collimating lenses perform the foregoing function, but at a cost. One aspect of the cost is that the light collection ability is less than that which would be desired. A further aspect of the cost is that the overall physical size of the collimating lens is larger than that which would be desired. Another disadvantage in existing collimating lenses in such scanning apparatus is the sensitivity of the lens characteristics to fabrication and alignment errors attendant the construction of such a visual display system. Also, prior art lenses of the foregoing type have had limited spectral bandwidth such that, upon a change in the light emission spectrum of the display, a further design of the collimating lens has been required. As an example of sensitivity to fabrication, it is noted that a typical construction of a collimating lens of the prior art has employed a field lens element which is closely positioned to the display being viewed. The close proximity to the display is attained by building the concave field lens as part of a composite structure including the display. Thus, it becomes necessary during fabrication of the display system to align the field lens element with the other elements of the collimating lens. It is difficult to maintain optical integrity of a lens, or lens assembly, wherein elements of the lens are mounted mechanically in different parts of the system, rather than as portions of a unitary optical assembly.